Functional safety of systems requires verification of equipment used for monitoring those systems. Monitoring equipment typically is digitally based, e.g., microcontrollers and microprocessors, but monitor analog parameters. This requires analog-to-digital conversion which typically is performed with an analog-to-digital converter (ADC). However, it is important that functional testing of the ADC be performed in the analog domain, e.g., analog inputs produce the correct digital outputs from the ADC. Also, the linearity of the ADC should be verified during the functional testing thereof. Thus, multiple voltage points may be used to confirm correct ADC transfer function. Verification of correct digital outputs from known reference voltages are desired.
FIG. 2 illustrations a prior technology schematic block diagram and graph for ADC self-testing using external voltage references. This is both complex and expensive since a number of voltage references, operational amplifiers and multiplexers must be used. Care must be taken in ensuring that the voltage references stay calibrated and the multiplexer switching does not distort or affect values of the voltage references.